The present teachings relate to systems and methods for forming a low stress electrical connection between laser diode systems and to forming a low stress connection that is capable of carrying a high electrical current.
A laser diode bar (LD-bar) is a monolithic semiconductor device, typically approximately 10 mm wide by approximately 150 μm thick, which contains numerous individual laser diodes (LDs) that are electrically in parallel. The top and bottom surfaces are the cathode and anode, respectively. The anode of the LD-bar is usually attached to a heat-sink since the light emitting regions, which generate heat, are generally much closer to the anode. The LD-bars are used individually as a linear source of high optical-power or in a stack as a two-dimensional source of high optical-power. Optical sources based on LD-bars are useful to optically pump other solid-state lasers, such as Nd:YAG laser, Yb-doped fiber laser, or as a direct energy source. Optical sources based on LD-bars can be useful when the optical power is coupled to an optical fiber. The efficiency of coupling optical power from a stack of LD-bars into an optical fiber depends on the placement accuracy of the LD-bars relative to each other.
For laser diode bars operating at high power in the continuous wave (CW) mode, the laser diode is generally attached to a water-cooled micro-channel heat-sink (MCH). The MCH has a water inlet and a water outlet and corresponding water seals to prevent the water from leaking. In one embodiment, O-rings are implemented as the water seals. LD-bars, each attached to a MCH, can be stacked so that the LD-bars are electrically coupled in series to form a two-dimensional array of light emitters. For stacking, the anode surface and the cathode surface of the MCH should be relatively flat and parallel.
The present high-power LD-bars generally have a slope efficiency of approximately 1 W/A. Additionally, the LD-bars are limited to operation at approximately 50-200 W due to thermal or reliability limitations to the device performance. For LD-bars emitting output power of approximately 50 W to 100 W, wire bonds are generally used to connect the LD-bar cathode to the MCH cathode. Such wire bonds are generally made of gold and the cathode surfaces are also generally made of gold. Attachment of the gold wire bond to the gold cathode surface is formed by thermo-compression. To avoid mechanical damage to the LD-bar from the thermo-compression attachment process, the gold wires are generally small in size, for instance, approximately 25-50 μm in diameter for round wires and approximately 25 μm×250 μm for ribbon bonds. For short lengths of wire, such as for lengths of several millimeters, each wire can carry 1-5 A of current, reliably. Thus, a 100 W LD-bar requires 20-100 wire bonds evenly distributed over the LD-bar cathode. For higher power and consequently, higher current, a thin metal foil, soldered to the LD-bar cathode, connects the LD-bar cathode to the MCH cathode. The solder should have a melting temperature that is lower than that of the solder used to attach the LD-bar to the MCH and should be sufficiently malleable at room temperature to avoid applying significant mechanical stress, due to coefficient of expansion mismatch between the metal foil and the LD-bar, which can be detrimental to LD-bar performance and reliability.
Accordingly, there is a need to provide techniques for forming a low stress electrical connection between the metal foil and the cathode of a high power LD-bar that is capable of carrying a high electrical current.